Preventing pulsations in centrifugal compressors.



M. I. NUSM. PREVENTING PULSATIONAS IN CBNTRIF'GAL COMPRESSORSAPYLIGATION FILED SEPT.11,1913.

Patented May 19, 1914.

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@TT I PATEN Fl MELACH I. NUSIIVI, OF LYNN, MASSACHUSETTS, ASSIGNOR TOGENERAL ELECTRIC COMPANY, A CORPORATION 0F NEW YORK,

noemen vSpeecation of Letters Patent.

Patented May ie, 1er/i.

Application filed September 11, 1913. Serial No. 789,259.

T0 all 'whom it maj/concern Be it known that I, Memoir l. Nostri, acitizen of the United States, residing at Lynn, county of Essex, Stateof Massachusetts, have invented certain new and useful Improvements inPreventing Pulsations in Centrifugal Compressors, otwhich the followingis a specification.

This invention relates to centrifugal air compressors of that type inwhich a rapidly rotating impeller throws the air through curved ortangential passages in a stationary diffusing ring surrounding saidimpeller. The action of these passages is to convert the velocity of theair into pressure, and when properly designed such machines may as in.the present case operate on what is known as a rising characteristic.This means that .the pressure does not remain constant for all volumesof air flowing through the intake, but varies with the volume delivered.ln order to secure an efficient and stable operation, the pressureshould be above a predetermined value, or so-called rated pressure. Ifit drops below this, the diusing ring causes the air column to fluctuateor pulsate. The pressure curve, plotted with varying volumetric outputas abscissas, shows that it starts at a point slightly below the desiredrated pressure and rises slowly as the volume increases until at abouthalf load it becomes slightly in eX- cess of said rated pressure, whenit suddenly jumps up to a higher pressure. Atter this it maintains afairly even pressure with increasing volume for some time, and then`radually drops to almost the rated pressure a't vfull load. This suddenjump in pressure is called the critical or breakdown point, and is thesource of great trouble when the compressor is trying to carry a lightload, the pressure fluctuating` violently and noisil y and setting updangerous vibrations and shocks in the machine.l and preventing thedelivery of a steady blast or' air, which is what is desired 'from anair compressor,

My invention aims to overcome these very objectionable surgii'igs orpulsations and insure a quiet, smooth operation of the. compressor atall loads. l' accomplish this by adjusting a multistage compressor todraw in a volume in excess of the breakdown load and then providingmeans foi-'blowing olf g such portion of the air as is not needed atthat point in the casing of the machine vwith the frame 22.

the object of my invention to locate the valve in such a position as torelease air from the casing at the lowest possible pressure, wherebythcstability of the compressor is regained with the least possible wasteof air, and a saving in power necessary for driving is effected. Thusthe preferable position of the valve in relation to the stages is aboutone-third the length of the casing away from the intake. In a three orfour stage machine, the valve would be in the first stage; in a ninesta-ge machine, at the third or fourth stage. rll'he blow-oft1 valve canbe operated either by hand or automatically; in the latter case it iscontrolled by a device which is responsive preferably to the airpressure in the, intake pipe of the com' presser.

In the accompanying drawing which illustrates one embodiment of theinvention, Figure l is a front elevation of a multistage centrifugalcompressor with t'he valve in position. Fig. 2 is a fragmentary sectionof the compressor showinga blow-o valve. Fig. El is a eross-section ofthe same showing in addition an automatic regulator. Fig. 4 illustratesa curve which givesthe pressure chariuftcristics.

Referring to the drawings, l0 indicates a casing for' a` multistagecentrifugal conipressor having an intake l1 and a discharge 12. Locatedin the casing is an impeller 1.3 secured to the shaft which is mountedin the bearings 15 and 1G. A chamber 17 is cast integral with the casingand is connected to the inner chamber 18 of the first stage ot thecompressor' by -a passage lt). Controlling this passage is a valve 2t)which is normally held on its seat by the spring 2l. One end of thespring 21 abuts against the rigid frame 92 and on the other against thewasher 23 which is tast on the spindle 24. For opening the valve 2O alever 25 is provided, ulcrunied on the bracket 26 which is cast Thellever 25 can be manually operated or connected to an automatic regulatorfor opening the valve under pulsating condition. When the valve 2O 1slifted from its seat air from the casing enters the chamber 17 and isled oft1 through the pipe Q7 and can be used for any desired purpose.

Located in the intake pipe 11 is a tube 28 acting like a Venturi tubewhich causes the aiif passing through it to increase in velocity anddecrease in pressure, thereby causing a difference in pressure from thatin the intake. These pressures are led olf through the pipes 29 and 30to the diaphragm 31 located in the casing 32. The high pressure when ator above the rated value acts against the diaphragm 31 and balances thelow pressure and the spring 33. Secured to the diaphragm 31 is a spindle34 the opposite end of which is secured to the bell crank 35 fulcrume-dat 36. The rod 37 connects the bell crank 35 and the lever 25. Theillustration is diagrammatic and the proportions of the parts areobviously incorrect from a practical standpoint.

When the discharge volume drops so low that the discharge pressure fallsto or near to the breakdown point, the difference in pressures on theopposite sides of the da# phragm 31 becomes less due to the lessenedrate of flow through the intake. The spring 33 is therefore able tooverbalance the higher pressure and forces the spindle 34 to the leftlifting the valve 2O to afford an additional discharge of air from thelow pressure stage of the machine. Whether accompli/shed by hand orautomatically, the effect of this is to permit la reduction below thebreakdown point of the volume of air passing out of the dischar e 12while maintaining a volume entering t rough the intake 11 which is abovethe breakdown. The result of this is to stop the pulsations, becauseactual experience indicates that the breakdownpoint occurs for a certainminimum How of air at the inlet of the machine, and if the flow at: thispoint can be kept above said minimum no trouble Will occur.

Fig.- 4 illustrates a curve which gives .the characteristics of acentrifugal compressor having rated volume and pressure and a dischargepressure indicated by the irregular line which cuts the line of ratedpressure. As the flow or volume of air is reduced below the rated value,the pressure at first rises slightly, and when the volume is furtherreduced to one-half or one-third the rated value, there is a' sudden droin pressure which is due to the fact that the internal assages in thediffusin ring are not suited) for eiiicient actionat lig ter loads.

In accordancelwith the provisions of the patent statutes, I havedescribed the principle of operation of my invention together with theapparatus which I now conslder to represent the best embodiment thereof;but I desire to have it understood that the a-ppaf ratus shown is onlyillustrative and that the invention can be carried out by other means.

'What I claim as rlew and desire to secure by Letters Patent of theUnited States, is

1. The method of reventing pulsations at light loads in a multi-stagecentrifugal air compressor, which consists in running said machine at arate of iiow above the required load, and blowing oil' the excess of airfrom a lower stage.

2. A multistage centrifugal air compressor having means for blowing offair from a lower stage.

3. A multistage centrifugal vair compressor having a blow-ofi'l openingin a lower stage of its casing and a valve controlling said opening.

4. Amultistage centrifugal air compressor having ya blow-0E opening ina'lower stage of its casing and a valve cont-rolling said opening, aspring normally holding said valve closed,` and means for opening saidvalve.

5. A multistage centrifugal air compressor having a blow-0H opening in alower stage of its casing and a. valve controlling saidl opening, aspring normally holding said valve closed, and means for opening saidvalve automatically when the air pressure approaches the breakdownpoint.

6. A multistage centrifugal air compressor having a blow-off opening ina lower stage of its casing, a valve controlling said opening, a springnormally holding said 'valve closed, and means for opening said valveautomatically when the air pressure approaches the breakdown point,comprising a device responsive to the flow of fluid in the intake of themachine, and operative connections between said device and the blow-offvalve.

7. A multistagel centrifugal compressor having a'blow-oif openingassociated with an intermediate pressure stagez a valve controlling t-heopening, and afdevice responsive -to changes inthe rate of How of theiluid passing ,through the compressor for opening the valve when theload on the compressor-`v has been reduced to or below the breakdownpoint.

In witness whereof, I have hereunto set my hand this sixth day ofSeptember 1913.

. MELACH I. NUSIM. lVitnesses p J A. MoMANUs,-Jr., FRANK G.. HATTIE.

